This invention relates to a method for submerged arc welding, and more particularly a four electrode welding process used for manufacture of light-wall steel pipe.
Large diameter steel pipes are typically formed by shaping a metal sheet into an O-shaped cross section by various mechanical means. Once shaped it is necessary to weld the seam along the edges of the sheet to seal the pipe. One method that has been widely used is the submerged arc welding process.
To improve production efficiency of the welding process, it is necessary to increase the welding speed. Once method of increasing the welding speed is the utilization of multiple electrodes. Methods have been developed utilizing multiple electrodes in arrangements of three, four, five, and more electrodes. Welding processes using three electrodes are disclosed in U.S. Pat. No. 2,436,387, U.S. Pat. No. 3,336,460 and U.S. Pat. No. 3,456,089. A welding process using four electrodes is disclosed in U.S. Pat. No. 4,442,340.
The welding speeds obtained with these processes are not efficient enough to meet current demands. Processes with five or more electrodes have very high deposition rates which lead to excessive weld reinforcement on light-wall pipe above 3.2 mm which is not allowable by most pipe specifications. In addition, excessive deposition does not add to the structural integrity of the product and wastes material. Another drawback to processes with five or more electrodes being used on light-wall material, is a high heat input which leads to melt through of the pipe.
There are many factors which limit the welding speed, the main factors are assurance of sufficient penetration depth and avoidance of welding defects. The most common type of welding defect associated with high welding speeds is undercutting. Undercutting is an unfilled area at the toe of the weld created by various sources including lack of filler material, adverse electromagnetic forces between arcs, and in general an unstable weld puddle. Undercutting is usually corrected by increasing the rate of weld deposition or by slowing the weld speed.
The two most common methods for increasing the deposition rate are increasing the input current to the electrodes and increasing the number of electrodes. The input current can be increased to only a certain threshold in multi-electrode welding. Above this threshold adverse electromagnetic interaction between the electrodes occurs. The higher the current input, the stronger the magnetic field interaction between the electrodes.
Significantly varying field interaction results in weld puddle instability. Thus, adding more electrodes would also increase the probability of adverse electromagnetic interference.
The goal therefore set forth is a welding process that provides increased welding speed, good weld puddle stability, beneficial rather than adverse electromagnetic interaction, good penetration capabilities, a limited number of electrodes, and capability of utilization on light wall metal material.
A process that has attempted to increase the efficiency of welding pipe is a four electrode submerged arc welding process disclosed in U.S. Pat. No. 4,442,340. This process increases the welding speed by applying alternating current to all four electrodes. Although this process displays improvement over previous art, it does not supply sufficient penetration capabilities at the higher speeds now required. One factor reducing the penetration capability of that the aforementioned patent is that it uses alternating current for the lead electrode which can not deliver the heat input efficiently enough to achieve the required penetration of the weld at the higher speeds demanded.
Increasing the current on the electrodes increases penetration. However increased current increases the rate of weld metal deposition. Further the increase in current required to obtain penetration is disproportional to the allowable increase in welding speed, consequently more deposition per linear length is deposited. Thus increasing linear weld deposition results in excessive weld reinforcement which is unallowable by most tubular product codes and specifications.
Another drawback to welding with increased current levels is an increased electromagnetic interaction between electrodes. When the electromagnetic fields between electrodes increase beyond a threshold point, the weld puddle becomes unstable resulting in undercutting.